This invention relates generally to transducers and particularly relates to a device for converting electric signals into acoustic signals or which may be driven partly or solely by acoustic waves.
Electrostatic transducers, that is loudspeakers or microphones, have been known in the art. They present a number of advantages. In particular, they are superior to most other loudspeakers for the reproduction of sound. They generate a clean sound and have a minimum of distortion. The reason is that the flexible diaphragm is moved uniformly over its entire surface by electrostatic force. Accordingly, the diaphragm doesn't have the tendency of conventional loudspeakers to be distorted or to break up into portions, each of which vibrates in a different mode.
An electrostatic loudspeaker is superb in the higher frequency range; however due to its mode of operation it needs to be rather large and requires considerable power to reproduce the low audio frequencies. For this reason a conventional loudspeaker to cover the entire audio range becomes large and hence cumbersome and is very directional.
Conventional electrostatic transducers are flat radiators. However, electrostatic transducers of the folded type have also been suggested in the U.S. Pat. Nos. to Williamson et al., 3,008,013 and 3,008,014. However, according to the Williamson et al. structure the fixed plates are disposed opposite each other and hence present a large capacitive load that does no useful work. In the conventional type of electrostatic transducers the sound must emerge at right angles to the planes of the conductive plates and therefore the plates need to be perforated. The edges of the perforations assume a charge which is higher than that of the flat surface of the plate and therefore the electrostatic pattern is both interrupted and made non-uniform. Also, at these perforations the insulation may break down. Finally, because the sound must pass through the perforations, the thickness of the insulation which may be used at these perforations is limited.
Furthermore, the large, flat loudspeakers in accordance with these patents provide a soundwave which is quite directional. This in turn may produce standing wave patterns in the room in which the loudspeaker is located so that localized loud and soft areas are created in the room due to interference patterns.
In an electrostatic loudspeaker the frequency response is proportional to the reciprocal of the square of the area of the diaphragm. On the other hand, the power required to drive the loudspeaker is directly proportional to the area of the fixed plates. This power is increased by the fixed capacitance which originates due to the fixed plates. By way of example, a loudspeaker having an area of 9 square inches produces sound at a frequency above one kilohertz (kHz) and requires one watt of driving power. On the other hand, in order to reproduce sound as low as 30 Hz an area of 100 sq. ft. is needed. In order to drive such a large loudspeaker a power of 1,000 watts is needed. Specifically, a large amount of power is required to drive a loudspeaker capable of reproducing audio frequencies, say below 40 Hz (Hertz) and specifically below 20 Hz.
It is accordingly an object of the present invention to provide an electrostatic loudspeaker where the area of the plates is substantially smaller than that of the diaphragm and where the plates are staggered and hence decoupled from each other, thereby to reduce the capacitance due to the plates and hence the necessary driving power.
Another object of the present invention is to provide an electrostatic loudspeaker of the folded type where one set of plates is made adjustable to adjust the tension of the diaphragm, thereby to vary the electrical resonance between a plate and its associated diaphragm portion so as to detune the transducer.
A further object of the invention is to extend the frequency range of a single electrostatic transducer to cover the entire audio range without requiring special units, which may be driven at low power and which may be constructed at reduced cost.
Still a further object of the invention is to provide an electrostatic transducer of the type discussed which is not directional, that is which creates a dispersive soundwave with superior acoustic coupling to the air and which provides a large area in a relatively small space.
Still another object of the present invention is to provide a transducer of the type discussed which can be driven partially or entirely by sound waves and which may therefore operate as a wave integrator.
Yet a further object of the present invention is to provide an electrostatic transducer having a diaphragm consisting of an electret, thereby to obviate the necessity of providing an electric battery.